An aluminum or tantalum solid electrolytic capacitor is used as a capacitor having high capacitance and low ESR (equivalent series resistance) to be used in various electronics equipment.
The solid electrolytic capacitor is constituted by an aluminum foil having fine pores in the surface layer or a tantalum powder sintered body having fine pores in the inside, which is serving as one electrode (electric conductor), a dielectric layer formed on the surface layer of the electrode, the other electrode (usually a semiconductor layer) provided on the dielectric layer, and a carbon layer and an electrode layer which are sequentially stacked on the other electrode.
For the semiconductor layer, an organic compound or an inorganic compound is used, specifically an electrically conductive polymer is preferably used in view of heat resistance and low ESR property of the capacitor produced thereof. The electrically conductive polymer is a polymer having high electrical conductivity of 10−2 to 103 S·cm−1, and the high electrical conductivity is exhibited by adding a compound so-called dopant to a polymer having a conjugated double bond (generally the polymer is an insulating body or has extremely low electrical conductivity). Examples of a specific method to form an electrically conductive polymer as a semiconductor layer includes a method of supplying an appropriate oxidizing agent or electrons in the presence of a dopant to a monomer capable of being polymerized to an electrically conductive polymer in the fine pores of the electric conductor thereby to polymerize the monomer and to obtain an electrically conductive polymer by incorporating the dopant at the polymerization of the monomer.
The carbon layer is provided for the purpose of protecting the semiconductor layer from various external stress and stably providing or taking electric charge from the semiconductor layer to the dielectric layer. The carbon layer uses a composition of at least one material selected from artificial graphite, natural graphite and carbon black to which a small amount of polymer materials as a binder is added appropriately.
The capacitor having a structure mentioned above is used in various circuits of electronic equipment, and various reliability tests are performed to judge if the initial performance of the capacitor is maintained for several years. One of these tests is a high temperature loading test of the capacitor. The test is to apply a rated voltage to a capacitor and leave it to stand at a high temperature for thousands of hours, and determine the acceptability of the capacitor based on whether the various performance values measured when the capacitor is returned to room temperature fall within a designated range. The capacitor is regarded to have a longer life, if it passes a test at a higher temperature and for a longer time of applying a voltage.